1. Field of the Invention
This invention relates to defect detection and inspection of knitted products utilizing light reflected off of the product, and more particularly, this invention is directed towards detection of defects in varying types of knitted products through the use of fiber optics and digital signal processors which reads reflected light.
2. Prior Art
Surface detection of light from optical components such as optical scanners and other such devices have been widely used in industry and in varying capacities. These detectors typically use electrical signals produced by an optical photo-sensor which may be digitized and processed as necessary in order to produce analytical data representing the scanned object. Most optical scanners use light emitted from a light source and an optical system in order to illuminate the object and focus a small area of light onto the illuminated object in combination with an optical photo-sensor. The most common type of photo-sensor device utilized in these systems is a charge coupled device or CCD. A CCD may comprise a large number of light sensitive cells or pixels each of which collects or accumulates electrical charges in response to the reflected light. Since the size of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on a surface which reflects the focused light. In most photo sensing devices, the voltage of a CCD pixel is measured and discharged at regular time intervals in order to provide sampling intervals sufficient to characterized the object being analyzed and also to provide data which may be accumulated and digitized over a specified time period. However, application of such photo sensing devices has proved difficult in the area of defect detection and inspection of knitted products.
Inspection of knitted products has been very difficult to apply to photo optic devices due to the various styles of knitted products which may vary the thickness of the inspected material. Further, this analysis is made even more difficult due to the different types of defects which may occur in knitting machines. Finally, the style of yarn utilized in these knitting machines can significantly affect the data obtained from optical detection and subsequent analysis.
Of the types of fabric knitted by circular knitting machines, fleece, rib and jersey are the most common and consequently defect detection of all three materials is significantly desired. Fleece material is made up of a three yarns requiring a facing yarn, a tie-in yarn and a backing yarn. Fleece material may have up to four or five differing styles. There is additionally a two-end fleece fabric wherein only a facing and backing yarn are utilized. Of the most common three-end yarns, 18 yarn is utilized for the bigger and heavier material wherein 100% cotton content is desired. Additionally, 24 yarn and 28 yarn materials may be utilized for blended polyester and cotton fabrics.
There is additionally jersey style and rib construction style fabric wherein dual yarn or single yarn materials are utilized to produce a thinner fabric. In jersey material construction for use in tee shirts and the like, photo-optic characteristics will be significantly different as compared to both rib and fleece material.
Any defect detection and inspection system must have the ability to differentiate between these three types of material in order to "see" defects occurring in the fabric. These defects may include end outs, dirty yarn, holes in the knitted fabric, heavy or light yarn, and a needle run. A secondary consideration for defect detection lies in the manner in which the material is knitted. Standard knitting machines feed the thread into the top of the circular knitting head producing a "tubular" knitted product which depends below the knitting head. This tubular knit rotates as the stitches are produced requiring the optical system to take this movement into account.
In an end out defect, a yarn is missing from the knitted material causing a spiral defect which is usually initiated by broken yarn and which must be detected on the single revolution pass of the tubular knit. The detection system must also have the ability to analyze the manufactured material and determine when dirty yarn, causing discoloration, has been used in the manufacturing process in order to alert the operator of the knitting machine. Holes in the knitted fabric may occur from machine defects or tearing of the knitted material leaving an actual opening in the tubular knit. Heavy or light yarn defects occur when the wrong gauge yarn is utilized in one of the feeding stations and which may be apparent during a single revolution of the tubular knitted fabric. These types of defects may also occur when the yarn becomes crossed and two yarns are found at one particular spot in the knitted material. Finally, a needle run occurs when a needle on the needle cylinder is broken causing a vertical line in the tubular knit fabric and is particularly hard to detect because the occurrence at the inspection station may be masked by the cotton content of the material. Other defects may be difficult to detect due to the fact that they only may be available for inspection at a single instance of a revolution at the inspection station.
Scanners which are presently available for use on jersey or fine material inspect the knit by applying a singular line of light in combination with a photo detector in order to obtain a higher resolution. These systems do not provide a high enough resolution to sort out runs or other anomalies, particularly in fleece material. This is a particularly sensitive issue when attempting to inspect fabric with prior art devices because of their use of a limited number of analog signals. This is additionally complicated in non-continuous defects which may only be viewed on a single revolution of the tubular knit fabric.
Continuous defects such as end outs, heavy or light yarn, or needle runs may be detected but are complicated somewhat by the varying types of fabric manufactured in these machines. The harder to detect faults in the fabric may be classified as spot faults in that they occur transversely as compared to the length way direction of the tubular fabric as it is knitted. Alternatively, the easier to detect continuous faults extend longitudinally along the tubular fabric. While all faults detected will not necessarily require stopping the high speed circular knitting machine, it may still be desirable to record the faults as they occur over a unit of time.
There are a variety of textile monitoring devices for tubular knitted fabric produced on a circular knitting machine. These devices utilize light sources and light detectors, usually photo cells, which are arranged in linear rows and which respond to variations in the reflectance of the fabric. Once the analysis reaches a pre-defined tolerance, the machine is turned off or a default detection signal is indicted. These systems typically utilize electro-optical sensors comprising infrared diodes and photo transistors arranged in alternate rows. However, as mentioned, these known devices fail to discriminate between types of defects or fail to have application on various types of product. U.S. Pat. No. 5,283,623 teaches a method and system for detection of faults in a length of textile fabric wherein six light sources are focused by a condenser onto the knitted structure of the length of fabric. Light reflected than passes back through a transparent disc to photo transistors matching the number of light emitting sources. The analysis method however requires scanning of zones on the fabric strip a number of times before a determination may be made as to defect or fault in the manufacturing process.
None of the prior art devices provide a means for defect detection and analysis for variable type knitting products or which accurately predict the type of defect occurring. Further, none of the prior art references utilize a large number of light emitting and detecting sources in order to increase the accuracy of the determination of fault with the material.